System for discriminating optical discs

ABSTRACT

A laser beam is focused at positions on an optical axis of the optical pickup by a focusing device, corresponding to kinds of the discs which are different in thickness. The focusing device is moved along the optical axis. A photodetector is provided for receiving a laser beam reflected from the disc and for producing a focus error signal. A detector is provided for detecting the difference between the focus error signals, caused by the difference in thickness of the discs and the difference in focal point of the laser beam, thereby discriminating the kind of the disc.

This is a divisional application of U.S. patent application Ser. No.08/869,741, filed Jun. 5, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disc discriminating systemin an optical disc player for discriminating various kinds of opticaldiscs such as CD and DVD (Digital Video Disc or Digital Versatile Disc).

A compatible single optical disc player which is capable of reproducinga CD (compact disc) and a LD (laser disc) has been popular. Since thethickness of the CD and the LD from the rear surface to the recordingsurface is the same (1.2 mm), it is possible to use an optical pickup inwhich a laser beam is focused on a focal point for reproducing both ofthe CD and the LD.

In order to discriminate the kind of the disc mounted on the player, thediameter of the disc is detected for discriminating the CD or the videodisc.

Recently, the DVD having a high recording density has been developed.Although the thickness of the CD from the surface to the recordingsurface is 1.2 mm, the thickness of the DVD is about 0.6 mm. However,the principle of a reading system of recording pits of the DVD is thesame as that of the CD. Therefore, a compatible single optical discplayer which is capable of reproducing the CD and the DVD has greatlybeen proposed.

In such a compatible CD/DVD player, in order to properly focus the beamfor reading the information on the recording surface of the disc, twotypes of lenses are provided, each having a focal length suitable for athickness of a corresponding disc. As another type of the compatibleCD/DVD player, a double focus lens using a hologram element is provided.

FIG. 8a shows an optical pickup having a double focus lens using ahologram element (diffraction grating).

The double focus lens comprises a diffraction grating 13, an objectivelens 11 disposed on the same optical path, and a collimator lens 14. Abeam of light is paralleled by the collimator lens 14 and divided intothree spectral beams of 0 order spectrum (having a 90 diffraction angle)and ±1 order spectrum by the diffraction grating 13 (−1 order spectrumis not shown). Since focal lengths of the 0 order spectrum and the +1order spectrum are different from each other, each of the 0 orderspectrum (hereinafter called zero-order spectrum) and the +1 orderspectrum (hereinafter called first-order spectrum) is focused on adifferent position of the same line.

Concretely, the zero-order spectrum is properly focused on the recordingsurface of the DVD, while the first-order spectrum is properly focusedon the recording surface of the CD.

In operation, when the pickup is gradually moved away from the disc, therecording surface is irradiated with the zero-order spectrum first. Afour-divided photodetector provided in the pickup (not shown) producesan S-shaped signal as a focus error signal. When the pickup is furthermoved away, the first-order spectrum reflected from the disc passes anoptical path of the zero-order spectrum, thereby forming a pseudospectrum. Thus, a pseudo S-shaped signal of the pseudo spectrum isproduced. Finally, an S-shaped signal of the first-order spectrum isproduced as a focus error signal.

FIG. 8b shows S shaped signals of the corresponding zero-order spectrum,pseudo spectrum, and first-order spectrum with respect to the movementsof the double focus lens of FIG. 8a. The spectral ratio of thezero-order spectrum and the first-order spectrum is set to 1:1.

As aforementioned, the zero-order spectrum achieves the optimum focus onthe DVD, while the first-order spectrum achieves the optimum focus onthe CD. If the amount of the S-shaped signal by the zero-order spectrumon the DVD is 100%, the amount of the signal corresponding to the CD issmall, less than 100%. Similarly, if the amount of the S-shaped signalby the first-order spectrum on the CD is 100%, the amount of the signalcorresponding to the DVD is small, less than 100%. Thus, in the doublefocus lens, the amplitudes of the respective focus error signals aredifferent from each other. The ratio of zero-order spectrum tofirst-order spectrum of the DVD, namely zero-order spectrum/first-orderspectrum is larger than the ratio of zero-order spectrum to first-orderspectrum of the CD, that is zero-order spectrum/first-order spectrum.

In the compatible CD/DVD player, a focus servo control system isprovided for each disc. In a focus servo control, the pickup isvertically moved against the disc, and a focus error (FE) signal isproduced for detecting a proper focal point for focusing in. However, inthe aforementioned player using the double focus lens, a plurality offocus error signals (S-shaped signals by zero-order spectrum, pseudospectrum and first-order spectrum) are obtained. Accordingly, a problemthat focusing in is achieved by an S-shaped signal detected firstarises. In other words, there may be occur an erroneous focusing by animproper spectrum.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical discdiscriminating system for an optical disc player which may properlydiscriminate a CD and a DVD, and select an optimum S-shaped signal in afocus error signal for focusing in.

According to the present invention, there is provided a system fordiscriminating various kinds of optical discs which are different inthickness, in an optical disc player having an optical pickup,comprising focusing means for focusing a laser beam at positions on anoptical axis of the optical pickup, the number of the positions beingcorresponded to the number of kinds of the discs, moving means formoving the focusing means along the optical axis, photodetector meansfor receiving a laser beam reflected from the disc and for producing afocus error signal, detector means for detecting the difference betweenthe focus error signals, caused by the difference in thickness of thediscs and the difference in focal point of the laser beam, and fordiscriminating the kind of the disc.

The focusing means comprises a diffraction grating and an objectivelens.

The detector means detects the difference in level of the focus errorsignal.

In another aspect of the invention, the detector means detects thedifference in timing of production of the focus error signal.

In a further aspect, the detector means detects the difference inposition of the focusing means when the focus error signal is produced.

These and other objects and features of the present invention willbecome more apparent from the following detailed description withreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an optical disc discriminating systemaccording to the present invention;

FIGS. 2a to 2 g are time charts showing signals for explaining anoperation of the system;

FIG. 3 is a flowchart showing an operation of the system;

FIG. 4 is a block diagram showing an optical disc discriminating systememployed for second and third embodiments of the present invention;

FIGS. 5a and 5 b are time charts showing signals for explainingoperations of the second and third embodiments;

FIG. 6 is a flowchart showing an operation of the second embodiment;

FIG. 7 is a flowchart showing an operation of the third embodiment; and

FIGS. 8a and 8 b are diagrams showing the principle of the system usinga hologram element as double focus lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 showing a system of a CD/DVD player according to thepresent invention, an optical disc 20 to be reproduced is mounted on theplayer and rotated by a spindle motor 21. An optical pickup 22 has adouble focus lens which has the same principle as FIG. 8a. The pickup 22emits a laser beam, and pits formed on a recording surface of the disc20 is irradiated with the beam for reading information recorded with thepits.

An output signal of the pickup 22 is fed to a focus error (FE) producingcircuit 23. The FE producing circuit 23 produces a focus error signal.After removing an unnecessary signal of a high frequency range from thefocus error signal, the focus error signal is applied to a peak holdingcircuit 24. The peak holding circuit 24 holds the maximum amplitudevoltage of the positive pole side of the focus error signal as a peakvoltage at a substantially constant value. The peak holding circuit 24applies the peak voltage to a microcomputer 25.

The microcomputer 25 produces a reset (RST) signal which is applied tothe peak holding circuit 24 for resetting the maximum voltage held inthe circuit. When a next focus error signal is applied to the peakholding circuit 24, a maximum amplitude voltage of the positive poleside of the next focus error signal is held as the maximum voltage.

The output signal from the pickup 22 is further applied to an RFOKproducing circuit 37. The RFOK producing circuit 37 receives an addedsignal of a four-divided photodetector. When the added focus signalexceeds a predetermined threshold value, the RFOK producing circuit 37produces an RFOK signal which is applied to the microcomputer 25.Namely, the production of the RFOK signal means the laser beam islocated at a position near focused position.

The focus error signal from the FE producing circuit 23 is furtherapplied to a focus zero cross (FZC) producing circuit 26. The FZCproducing circuit 26 is a zero cross comparator comprising anoperational amplifier and produces a focus zero cross detecting signalin the form of pulses. The production of the FZC signal means that thespectrum is focused. The FZC signal is applied to the microcomputer 25.

The focus error signal from the FE producing circuit 23 is also appliedto a fixed contact of a loop switch 27. The loop switch 27 is operatedby an FE shunt signal from the microcomputer 25.

A movable contact of the loop switch 27 is connected to a focus servocontrol circuit 28 which is connected to the microcomputer 25. The focusservo control circuit 28 is further connected to a focus servo coildrive circuit 29 which is connected to an actuator coil 30.

When the loop switch is closed (ON-state), a focus servo loop comprisingthe FE producing circuit 23, microcomputer 25, focus servo controlcircuit 28, focus servo coil drive circuit 29 and actuator coil 30 isclosed. Thus, the focus error signal is applied to the focus servocontroller 28 for automatically controlling the focus servo.

When the loop switch 27 is opened (OFF-state), the microcomputer 25operates the focus servo coil drive circuit 29 to produce the drivevoltage for driving the actuator coil 30. Thus, the lens in the pickupis moved downward and upward at a predetermined speed for discriminatingthe disc.

The microcomputer 25 is further connected to a RAM 32, a ROM 33 and amotor drive circuit 31. The RAM 32 stores data of signals (time andvoltage) detected by circuits, and set values obtained by the detectedsignals. The ROM 33 stores data of set values necessary for the system.The motor drive circuit 31 produces a drive signal for driving thespindle motor 21.

FIGS. 2a to 2 g show time charts of signals for explaining the operationof the system. As aforementioned, the double focus lens in the pickup 22has the same principle as FIG. 8a, and the spectral ratio of thezero-order spectrum and the first-order spectrum is 1:1. Thus, when thelens is vertically moved, the relationship between the maximumamplitudes of the S-shaped signals of zero-order spectrum andfirst-order spectrum is represented as follows.

(FED0/FED1)>(FEC0/FEC1)

where

FED0: maximum amplitude voltage of the S-shaped signal of the DVD by thezero-order spectrum

FED1: maximum amplitude voltage of the S-shaped signal of the DVD by thefirst-order spectrum

FEC0: maximum amplitude voltage of the S-shaped signal of the CD by thezero-order spectrum

FEC1: maximum amplitude voltage of the S-shaped signal of the CD by thefirst-order spectrum

FIG. 2a shows a focus drive voltage SFD for representing a positionvoltage of the lens. A mark UH is the highest position of the lens (theclosest position of the lens to the optical disc 20), and a mark UL isthe lowest position of the lens (the farthest position of the lens fromthe disc).

FIGS. 2b and 2 c show voltages of the focus error signals (FE) having Sshape for the CD and the DVD, respectively, obtained during the upwardand downward movements of the lens. A mark TH1 is a threshold of thevoltage stored in the ROM 33 for detecting the S-shaped signal. The TH1is set at least smaller than the maximum amplitude voltages obtained bythe zero-order spectrum of the CD and DVD. The FZC signal is produced bycomparing with the TH1. A mark TH2 is a threshold set to a value betweenthe maximum amplitude voltage of the S-shaped signal of the CD by thefirst-order spectrum FEC1 and the maximum amplitude voltage of theS-shaped signal of the DVD by the first-order spectrum FED1(FEC1>TH2>FED1). The TH2 is stored in the ROM 33 or obtained bymultiplying the S-shaped signal by the zero-order spectrum and acoefficient together. In this state, influence of the reflectance of thedisc is prevented.

FIG. 2d shows the zero cross detecting pulse signal FZC for each of CDand DVD obtained by the FZC producing circuit 26 when the zero cross ofthe S-shaped signal is detected.

FIG. 2e shows the reset pulse signal RST for each of CD and DVD appliedfrom the microcomputer 25. FIG. 2f shows output signals of the peakvoltages from the peak holding circuit 24 for CD and DVD. A mark V1 is awaveform and a measured point of the maximum amplitude voltage of theS-shaped signal by the zero-order spectrum. A mark V2 is a waveform anda measured point of the maximum amplitude voltage of the S-shaped signalby the first-order spectrum.

In FIGS. 2e and 2 f, marks T1 and T2 are times measured by a timerprovided in the microcomputer 25. As shown in FIG. 2e, the time T1represents the time when the peak voltage is reset after the peakvoltage of the zero-order spectrum is measured by the FZC formed by theS-shaped signal of the zero-order spectrum. For example, if the pseudoS-shaped signal is large, and if the time is set to the middle betweenthe pseudo spectrum and the first-order spectrum after the peak voltageis measured to pass the pseudo S-shaped signal, the pseudo S-shapedsignal is masked. The time T2 represents the time passing thefirst-order spectrum from the FZC and measuring the peak voltage offirst-order spectrum, or the time until the lens reaches the lowestposition UL.

FIG. 2g shows the RFOK signal of each of CD and DVD, which will bedescribed hereinafter.

The operation of the system will be described with reference to theflowchart of FIG. 3.

At a step S1, the optical disc 20 is loaded on the disc player. At astep S2, the loop switch 27 is opened (OFF-state), and the double focuslens in the pickup 22 is moved up to the highest position UH. At a stepS3, the peak voltage is reset, and the lens is moved down from thehighest position UH to the lowest position UL at a predetermined setspeed. During the downward movement of the lens, the pickup 22 producesthe focus error signal.

At a step S4, the maximum voltage of the FE of the S-shaped signal ismonitored by the FZC signal if the FE exceeds the threshold TH1. WhenFE≧TH1, the timer is set at a step S5. At a step S6, the peak voltage ofthe positive pole side of the S-shaped signal obtained first by the peakholding circuit 24 for a predetermined time is stored in the RAM 32 asthe peak voltage V1. At a step S7, a time T measured by the timer iscompared with the set time T1 stored in the ROM 33. If T≧T1, themicrocomputer 25 applies the reset signal RST to the peak holdingcircuit 24 for discharging the peak voltage V1 held therein to be zeroat a step S8. If not at the step S7, the timer is set until T≧T1 isobtained.

At a step S9, the measured time T is compared with the set time T2stored in the ROM 33. If T≧T2, when the timer counts the time T2 at astep S10, the peak voltage held in the peak holding circuit 24 is storedin the RAM 32 as the peak voltage V2. AT a step 11, the peak voltage V2is compared with the threshold TH2 stored in the ROM 33. If V2≧TH2, itis determined that the disc 20 is CD at a step S13. If not, it isdetermined that the disc 20 is DVD at a step S12.

At a step S14, the position of the lens is monitored until SFD≧UL. WhenSFD≧UL, the timer is reset at a step S15. At a step S16, the lens ismoved up. At a step S17, the position of the lens is monitored untilSFD≧UH. When SFD≧UH, the lens is moved down at a step S18. At a stepS19, the S-shaped signal obtained first is monitored by the FZC untilthe FE exceeds the threshold TH1. When FE≧TH1, and when it is determinedthat the disc determined at the step 11 is CD at a step S20, the timeris set at a step S21. If the disc is DVD at the step S20, the programgoes to a step S23.

At a step S22, the measured time T is monitored until T≧T1. When T≧T1,the microcomputer 25 is operated to close the loop switch 27 at the stepS23. At a step S24, when the zero cross signal FZC is produced, it isdetermined that the laser beam is focused. At a step S25, a seriesoperation for disc discrimination is terminated.

In the step S11, although the disc is discriminated by V2≧TH2, there isanother method for discriminating the disc.

As aforementioned, since (FEC0/FEC1)<(FED0/FED1), the disc may bediscriminated by the peak voltages V1 and V2 of the peak holding circuit24. If a certain value β is set to (FEC0/FEC1)<(FED0/FED1), and the discis discriminated with the value β, the disc can be discriminated withoutinfluenced by the reflectance of the disc.

Namely, as shown in FIG. 2f, if the peak voltage of the S-shaped signalby the zero-order spectrum is V1 and the peak voltage of the S-shapedsignal by the first-order spectrum is V2, it is determined whetherV2×β>V1 at the step S11. When V2×β>V1, it is determined the disc is CDat the step S13. If not, it is determined the disc is DVD at the stepS12.

FIG. 2g shows the RFOK signal of each of CD and DVD.

In the embodiment, timings for starting the measurement of times T1 andT2 may be determined by the RFOK signals, not the FZC of the zero-orderspectrum. Furthermore, it is possible to determine the timings when thepeak voltage output exceeds the TH1.

The lens may be moved from the lowest position to the highest position.In this method, although the S-shaped signal becomes the order offirst-order spectrum, pseudo spectrum and zero-order spectrum, it ispossible to discriminate the disc in the same manner.

In the above described embodiment, it is possible that when the maximumvoltage V2 is detected, it is determined that the lens is positioned atthe lowermost position, or may be moved to the uppermost position.

Although the timing for resetting the focus is set to the middle betweenthe pseudo S-shaped signal and the S-shaped signal by the first-orderspectrum, it is possible to reset after the V1 is measured if the pseudoS-shaped signal is small.

In order to prevent the influence of the recording surface of the disc,the disc may be rotated at a predetermined speed at the step S2.

FIG. 4 shows a system employed for second and third embodiments. In thesystem, an equalizer (EQ) 34 for correcting the drive voltage applied tothe actuator coil 30, and a comparator 36 having a reference voltagesource 35 as a reference voltage for detecting voltage are provided, andthe peak holding circuit 24 of the first embodiment is omitted. Otherstructures are the same as those of the first embodiment, and the sameparts are identified with the reference numerals as FIG. 1, and thedescriptions thereof are omitted.

The EQ 34 is operated for linearly holding a relationship between thedrive voltage of the actuator coil 30 and the position of the lens. Anoutput voltage of the EQ 34 is applied to an input terminal of thecomparator 36 and the other input terminal is connected to the referencevoltage source 35.

FIGS. 5a and 5 b show signals for operating the system for second andthird embodiments. A mark FD is a focus drive voltage of the drivevoltage of the actuator coil 30 converted by the EQ 34. The FD islinearly changed with respect to the position voltage SFD.

In FIG. 5a, if the lens is further lowered from the position where thereference voltage E is obtained, times when the S-shaped signals areproduced first by the pickup 22 by the respective DVD and CD aredifferent from each other. Therefore, a mark TD1 is a detecting time ofthe DVD and a mark TC1 is a detecting time of the CD. The detectingtimes TD1 and TC1 are stored in the ROM 33 as set values.

In FIG. 5b, the FD produced at the detecting time TD1 or TC1 is storedin the ROM 33 as a set value. Marks E1 and E2 are voltages thereof.

The operation of the system of the second embodiment will be describedwith reference to FIG. 6.

At a step S1, the disc 20 is loaded on the disc player. At a step S2,the lens is moved up to the highest position UH. At a step S3, the lensis moved down from the highest position UH to the lowest position UL ata predetermined set speed.

At a step S4, the FD is monitored, and when FD≧E, the timer is set at astep S5. At a step S6, an S-shaped signal detected first is monitored bythe FZC. When EF≧TH1, the timer is stopped at a step S7. At a step S8,the time during the timer is set is stored in the RAM 32. At a step S9,the time T is compared to the set time TD1 stored in the ROM 33. Thetime T is set to TC1<T<TD1. If T≦TD1, it is determined that the disc isDVD at a step S10. If not, it is determined that the disc is CD at astep S11.

Programs in steps S12 to S23 are the same as those in the steps S14 toS25 of FIG. 3 of the first embodiment. Thus, the descriptions thereofare omitted.

The operation of the system of the third embodiment will be describedwith reference to FIG. 7.

At a step S1, the disc 20 is loaded on the disc player. At a step S2,the lens is moved up to the highest position UH. At a step S3, the lensis moved down from the highest position UH to the lowest position UL ata predetermined set speed. At a step S4, the S-shaped signal obtainedfirst is monitored by the FZC until the FE exceeds the threshold TH1.When FE≧TH1, the FD is stored in the RAM 32 at a step S5.

At a step S6, the FD (E1 or E2) stored in the RAM 32 is compared with areference voltage E0 stored in the ROM 33. The reference voltage E0 isset to the value to meet E1<E0<E2. When E0≦E1, it is determined that thedisc is CD at a step S7. If not, it is determined that the disc is DVDat a step S8.

Programs in steps S9 to S20 are the same as those in the steps S14 toS25 of FIG. 3 of the first embodiment. Thus, the descriptions thereofare omitted.

In the second and third embodiments, the FZC of the zero-order spectrumfor monitoring the programs may be effective by the RFOK signal shown inFIG. 2g of the first embodiment, or by the peak voltage output exceedsthe TH1.

As aforementioned, the lens may be moved from the lowest position to thehighest position.

The EQ 34 is provided for removing noises of the high frequency range ofthe focus drive signal. If there is no noises, the EQ 34 may be omitted.

In order to prevent influences of inclination and vibration of the disc,the disc may be rotated at a predetermined speed at the step S2.

In accordance with the present invention, in the first embodiment, theobjective lens for focusing the beam on the disc is moved from thepredetermined position, thereby obtain the focus error signal. When thefirst focus error signal appears, the time is measured. When the timebecomes the predetermined time, the level of the focus error signal atthe predetermined time is compared with a set reference level. Thus, itis possible to discriminate CD and DVD, and to focus in the properS-shaped signal in accordance with the level of the focus error signaland the measured time.

Furthermore, in the second and third embodiments, when the first focuserror signal appears, the focus drive voltage is detected. The detectedfocus drive voltage is compared with the reference voltage and the timefor detecting the voltage is compared with a reference time. Thus, it ispossible to discriminate CD and DVD, and to focus in the proper S-shapedsignal in accordance with the focus drive voltage and the measured time.

While the invention has been described in conjunction with preferredspecific embodiment thereof, it will be understood that this descriptionis intended to illustrate and not limit the scope of the invention,which is defined by the following claims.

What is claimed is:
 1. A method for discriminating at least two kinds ofoptical discs which are different in thickness, in an optical discplayer having an optical pickup (22), a focus error producing circuit(23) and a peak hold circuit (24), comprising the steps of: resetting avalue in the peak hold circuit; moving the pickup toward an optical discto a first predetermined position (UH) along an optical axis of thepickup; irradiating a laser beam from the pickup on a recording surfaceof the optical disc; moving the pickup toward a second predeterminedposition (UL) from the first predetermined position at a predeterminedset speed; receiving a laser beam reflected from said recording surfaceand producing a focus error signal (FE); holding a peak value of thefocus error signal in the peak hold circuit; starting to count a time(T) from when a value of the focus error signal becomes equal with afirst reference value (TH1); resetting the value in the peak holdcircuit when the time becomes equal with a first predetermined time(T1), and continuing to hold the peak value in the peak hold circuit;detecting a difference between the held value in the peak hold circuitwhen the time becomes larger than a second predetermined time (T2) and asecond reference value (TH2), and discriminating the kind of the discbased on the detected difference which is different in accordance withthe kinds of discs.